Develop compress closenotifier

This allows client request cancellations to be passed through the http.CloseNotifier interface. 

If the request has been cancelled before the result reaches the compressor, it will not compress the result, and you will save the CPU, that would otherwise be used to compress the unused result.

Also, this completely avoids inserting the compressor, if it is disabled.

Note: To test this, you must be in a production environment, as the revel development proxy will not pass cancellations through .
For type assertions I think `ok` is pretty widely accepted. e.g. http://golang.org/src/pkg/net/dial.go